The present invention relates to a semiconductor laser apparatus.
Since semiconductor laser apparatuses currently used are compact and efficient, a rapid improvement in optical sensing techniques such as an optical recording technique has been achieved. However, in consideration of a sensing technique using polarized light, a light source capable of polarization control is indispensable.
Conventional polarization control in semiconductor laser apparatuses requires an optical system for controlling a reflection loss by using 35 polarizers and the like. For this reason, the size of each element is increased, or an unstable operation results from a positional shift in components.
Consider oscillation mode control in the semiconductor laser apparatuses. Generally, oscillation is produced in only the TE mode (in which electric field components are parallel to the active layer), or in only the TM mode (in which magnetic field components are parallel to the active layer) by applying a tensilely strain to the active layer to increase the gain of the TM mode.
FIGS. 29A and 29B show a conventional semiconductor laser apparatus. Referring to FIGS. 29A and 29B, reference numeral 1 denotes a semiconductor substrate; 2, an active layer consisting of GaAs; 3, an upper cladding layer consisting of AlGaAs; 4, a lower cladding layer consisting of AlGaAs; 5, an insulating layer consisting of SiO.sub.2 ; 6, an upper electrode; 7, a lower electrode; 8 and 9, resonator mirrors; and 10, an optical waveguide stripe.
In this semiconductor laser apparatus, since the active layer is a bulk member, there is no difference between the gain obtained by current injection in the TE mode (in which an electric field is parallel to the active layer) and that in the TM mode (in which an electric field is perpendicular to the active layer). However, the reflectance of each resonator mirror is higher in the TE mode than in the TM mode. For this reason, the semiconductor laser mainly oscillates in the TE mode. In this case, the light intensity of the TM mode is not more than 1/100 that of the TE mode.
If a quantum well structure having no strain or having a compressively strain is used for the active layer, since electrons in a conduction band are mainly combined with heavy holes in a valence band, the gain obtained by current injection in the TE mode is higher than that in the TM mode. Owing to this high gain in the TE mode as well as the effect of the reflectance of each resonator mirror, the laser apparatus is caused to oscillate in the TE mode.
In contrast to this, if a quantum well layer having a stretching strain is used for the active layer, since electrons in a conductive band are mainly combined with light holes in a valence band, the gain obtained by current injection in the TM mode is higher than that in the TE mode. If this effect is greater than the effect of the reflectance of each resonator mirror, the laser apparatus is caused to oscillate in the TM mode.
As described above, the conventional semiconductor laser apparatuses oscillate either in the TE mode or in the TM mode.
In digital optical communications in which polarized light beams are assigned to information "0" and information "1", polarization interference optical systems for heterodyne detection, optical sensors based on differences in reaction to polarized light, and the like, both the TE and TM modes are required. In such a case, if a conventional semiconductor laser apparatus is used, a wave plate for converting polarized light of the TE mode into that of the TM mode is required. Generally, a wave plate is made of an expensive dielectric crystal having birefringence, and is inserted, as a discrete component, in an optical system. Therefore, the following problems are posed, for example:
(1) In order to convert the polarization state of one light beam, a mechanism for rotating or moving the wave plate is required. In addition, the conversion speed is low.
(2) With a reduction in the size of the optical system, a cumbersome positioning operation is required for assembly, resulting in an increase in the cost of the optical system.